In 2008 a U.S. Pat. No. 7,450,231 to Johs et al. issued that described a substantially achromatic Retarder. In March 2011 a U.S. Pat. No. 7,907,280 issued to a method of manufacturing said retarder issued, and in June of 2013 a Patent for a mounting for said retarder, U.S. Pat. No. 8,462,341 to He et al. issued. Said 231, 280 and 341 Patents are entered by reference herein. Additional Patents to said retarder etc. have issued in foreign forums as well.
The retarder previously taught in the identified Patents provided that four total internal reflection occur, each of which entered some retardance into a beam of electromagnetic radiation. To improve the achromaticity of the entered retardance, the surfaces of the retarder elements at locations thereof at which total internal reflectance occurred, were coated with MgF2. It has been found, however, that while this coating of MgF2 on the appropriate surfaces improved achromaticity, it also causes reduction in beam intensity. This is a major concern in some applications where insufficient beam intensity can cause degradation of the ability to investigate samples. Also provided was a dual wedge arrangement following the retarder element per se., that facilitated correcting for beam deviation and displacement.
A need remains for a substantially achromatic retarder system that provides substantially achromatic entry of phase retardation between orthogonal components of a polarized electromagnetic beam, over a specified wavelength range, without significantly and adversely affecting the intensity of said beam.